1. Field of the Invention
The present invention relates to an image data processing apparatus that compresses and stores image data, and expands and restores it in outputting, an image forming apparatus provided with the same, an image data processing program, and an image data processing method.
2. Description of the Related Art
There is known a device treating image data, such as compressing acquired image data and storing it as compressed data and expanding it at the time of use, to save a storage capacity. In many cases, the device that performs compression and expansion processing of the image data has a compression circuit (compression unit) and an extension circuit (expansion unit) to perform compression and expansion processing. Even in a case of the device that performs compressing and expansion processing by software, processing time appropriate for such a processing is required.
Meanwhile, in some of the aforementioned devices, the image data must be outputted at a prescribed speed when stored image data is used. For example, in an electrophotographic image forming apparatus, an electrostatic latent image is formed by sequentially exposing a surface of a photoconductor that moves at a constant speed, by dots of light corresponding to each pixel of the image data. The expansion processing of the compressed data must be completed by the time of performing exposure corresponding to each dot. For example, in a high speed image forming apparatus (high speed machine) in which the number of printed sheets per every minutes is 50 sheets or more, a moving speed of a photoconductor surface is faster than the moving speed of a middle/low speed machine. Also, in an expansion processing unit of the image data, a processing speed is required in matching with this unit. However, in the high speed machine, in some cases, the aforementioned moving speed exceeds a processing capability of an expansion unit single body. Therefore, there is known a device in which a plurality of expansion units are operated in parallel and an apparent expansion speed is increased. Further, there is known a device in which the number of the compression units and the expansion units is switched based on a data quantity of data to be processed (refer to, for example, Japanese Unexamined Patent Application Publication No. 11-41429).
The compressed data generated by compressing the image data as one block can be expanded only by one expansion unit. Accordingly, in order to operate a plurality of expansion units in parallel and realize high speed processing, a structure of compressed data needs to be devised at the time of compression. When the compression processing and expansion processing of the image data are always performed by specific compression unit and expansion unit, the structure of the compressed data may be simple.
However, for example, in some cases, there are a plurality of kinds of output destinations, and a speed to output the image data is different in each kind. As a specific example, there is given a case that the image data processing apparatus of the same type is mounted on a plurality of models of image forming apparatuses having different print speeds respectively. In addition, transfer of the compressed data is sometimes performed among a plurality of devices having different print speeds, respectively, such as a function of a dispersion treatment of one print job by a plurality of image forming apparatuses, namely, a case of executing a so-called tandem process. Alternately, there is a case that the image data obtained by a first device is held in this device and according to an instruction from a user, the data held by a second device connected via a network is transferred and is printed. Alternately, there is also such a case that the image data obtained by the first device is transferred to the second device via the network and is held by the second device, and according to the instruction from the user, the held data is printed by the second device. Such a processing form is a so-called one aspect of a filing function.
In such a case also, according to a performance of the device performing the expansion processing, it is remarkably preferable to have a data structure capable of changing an operation state such as with or without a parallel operation of the expansion unit or the number of the expansion units that perform the parallel operation. However, the structure of the data capable of realizing such a function is not simple.